feat(chat): chat limit — bulk quarter-unload, 75% fresh-load window, show-earlier page-in

Long transcripts no longer grow unbounded: past the chat limit (default 256
chunks, localStorage dispatch.chatLimit) the oldest ceil(limit/4) committed
chunks are unloaded in ONE bulk pass — never one-per-delta (old Dispatch's
scroll-jump-per-step bug) — and only while the reader is stuck to the bottom
(scrolled-up readers defer the trim; it catches up in whole quarters). A fresh
page load windows to the newest floor(0.75*limit). Unloading is purely local
(IndexedDB cache + server keep everything); a hiddenBeforeSeq watermark keeps
history merges from resurrecting unloaded chunks, and a 'Show earlier messages'
affordance pages a quarter back in from the cache with scroll-anchor
preservation. Thinking-collapse render keys stay stable across trims via a
hiddenThinkingCount ordinal base.

- core/chunks/trim.ts: pure policy (trim/window/restore/normalize) + tests
- chat store: chatLimit + canUnload deps, windowed load, showEarlier()
- composition root: dispatch.chatLimit localStorage knob + unload gate wired
  to smart-scroll isAtBottom()
- backend CR-5 OPENED (not a blocker): ?limit=/?beforeSeq= on
  GET /conversations/:id (courier backend-handoff-chat-limit.md)
- scripts/live-probe.ts: fix pre-existing stale TurnMetricsEntry reads
  (m1.usage -> total.usage) that crashed the probe; 17/17 live checks pass

1 files changed, 39 insertions, 0 deletions

diff --git a/src/features/chat/ui.test.ts b/src/features/chat/ui.test.ts
index 278b2cf..7174821 100644
--- a/src/features/chat/ui.test.ts
+++ b/src/features/chat/ui.test.ts
@@ -41,6 +41,45 @@ describe("ChatView", () => {
 		expect(screen.getByText("Hello!")).toBeInTheDocument();
 	});
 
+	it("shows the show-earlier button only when earlier history is unloaded, and pages it in", async () => {
+		const chunks: RenderedChunk[] = [
+			{ seq: 26, role: "user", chunk: { type: "text", text: "later" }, provisional: false },
+		];
+
+		let resolveEarlier: (() => void) | undefined;
+		const onShowEarlier = vi.fn(
+			() =>
+				new Promise<void>((resolve) => {
+					resolveEarlier = resolve;
+				}),
+		);
+
+		render(ChatView, { props: { chunks, hasEarlier: true, onShowEarlier } });
+
+		const button = screen.getByRole("button", { name: /show earlier messages/i });
+		const user = userEvent.setup();
+		await user.click(button);
+
+		expect(onShowEarlier).toHaveBeenCalledTimes(1);
+		// While the page-in is awaited the button is disabled (no double-fire).
+		expect(screen.getByRole("button", { name: /loading earlier messages/i })).toBeDisabled();
+
+		resolveEarlier?.();
+		await vi.waitFor(() => {
+			expect(screen.getByRole("button", { name: /show earlier messages/i })).toBeEnabled();
+		});
+	});
+
+	it("hides the show-earlier button when nothing is unloaded", () => {
+		const chunks: RenderedChunk[] = [
+			{ seq: 1, role: "user", chunk: { type: "text", text: "all here" }, provisional: false },
+		];
+
+		render(ChatView, { props: { chunks, hasEarlier: false, onShowEarlier: vi.fn() } });
+
+		expect(screen.queryByRole("button", { name: /show earlier/i })).not.toBeInTheDocument();
+	});
+
 	it("renders tool-call chunks", () => {
 		const chunks: RenderedChunk[] = [
 			{